Acute Respiratory Distress syndrome (ARDS) can arise from a number of insults and leads to high rates of morbidity and mortality. This condition is characterized by epithelial and endothelial cell injury in the alveolar septa with a significant inflammatory response. ARDS frequently develops in critically ill patients subjected to treatment with high partial pressures of oxygen. Exposing mice for prolonged periods of time to 100% oxygen provides a useful model for testing specific hypotheses concerning ARDS pathogenesis. Reactive oxygen species contribute to the pathogenesis of this disease. The enzyme extracellular superoxide dismutase (EC-SOD) is highly expressed in the lung and is the major antioxidant enzyme in the extracellular matrix. EC-SOD has been shown to significantly protect mice against hyperoxic lung injury, but the mechanism of injury is still unclear. EC-SOD has a heparin-binding domain that regulates its biodistribution in the extracellular matrix. This domain is sensitive to proteolysis, and proteolytic removal of the heparin-binding domain may be a primary mechanism to regulate EC-SOD levels in extracellular spaces. We found that EC-SOD strongly co-localizes with type I collagen in the alveolar septa of the lung. Collagen is sensitive to superoxide-mediated fragmentation. Furthermore, EC-SOD is a potent regulator of nitric oxide activity and prevents the formation of peroxynitrite, which can also leads to collagen fragmentation. Notably, collagen fragments are potent chemoattractants and activators of neutrophils. We hypothesize that EC-SOD's association with type I collagen in the lung may prevent inflammation by inhibiting oxidative fragmentation of collagen. We also hypothesize that proteolytic clearance of EC-SOD during pulmonary oxygen toxicity may contribute to the pathogenesis of this disease by augmenting collagen fragmentation and inflammation. The proposed studies will provide novel insight into oxidative and signaling mechanisms that contribute to acute lung injury and point to new targets for pharmacologic intervention.